Avoid overwriting dicts.dat if it is unchanged

[SReicheltPTV]

This fixes another concurrency problem that is similar to #1600 and which I hadn't noticed at the time. At the moment, dicts.dat is overwritten every time a Python instance accesses an interface for the first time, even if the cache files for this interface already exist and dicts.dat is already up to date. This causes problems if another Python instance tries to read or write dicts.dat at the same time. So I changed the code to update dicts.dat only if its contents were actually changed, making this far less likely.

[mhammond]

Thanks!